1. Field of the Invention
The present invention relates to a magnetic field molding device, method for producing ferrite magnet and die which can be used for them.
2. Description of the Related Art
Ferrite (sintered) magnets are massively going into widely diversified areas, e.g., vehicles, home electric/electronic appliances and industrial machines, because of their improved properties and relative inexpensiveness.
Ferrite magnets are produced by a series of steps of calcining a raw material mixture with a given composition into a ferrite state, milling the resulting calcined body into a fine ferrite powder of submicron size, compression-molding the powder into a molded body using a die in a magnetic field (hereinafter referred to as magnetic field molding), and sintering the molded body into a ferrite magnet.
The processes for magnetic field molding fall into two general categories; dry process wherein the powder is molded as a dried material and wet process wherein the powder is molded as a slurry.
The wet magnetic field molding involves a problem of decreased production yield resulting from cracking or the like of the molded body, unless the slurry is dehydrated enough to remove its water content.
Therefore, there has been proposed a technique for improving the dehydration properties of the slurry in which the slurry be heated before it is injected into a die to reduce its viscosity and thereby to improve its dehydration properties, as disclosed in, e.g., Patent Documents 1, 2 and 3.
Patent Document 1 proposes a technique in which a heating device for heating a slurry is provided between a die assembly and a pressure pump for pumping the slurry to the die assembly.
This technique, however, which uses an electric heater tube or water bath as the heating device, involves a problem of needing a long heating time. Patent Document 2, in an attempt to solve the above problem, proposes a technique in which microwaves are used to uniformly heat the slurry in a shorter time.
Patent Document 3 proposes the following techniques. That is, the slurry in a tank is directly heated by a pipe heater or the like before being injected into the die; or indirectly heated by hot water or the like circulating over the tank; or the slurry is flowing in a pipe connecting the tank to the die, into which it is to be automatically injected, and the slurry is heated by heating the pipe periphery. Thereby, the slurry is kept from 40 through 90° C.
However, the inventors of the present invention have found that injection of the heated slurry into a die causes problems resulting from decreased temperature of the slurry and consequently increased viscosity of its dispersion medium, because it is quenched by the die or the like.
The technique disclosed by Patent Document 3 has the essence of keeping the slurry from 40 through 90° C. in a die, for which it is heated while it is held in a tank before being sent to the die directly by a pipe heater of the like or indirectly by hot water or the like circulating over the tank, or while it is flowing in a pipe to the die, into which it is to be automatically injected, by heating the pipe periphery, as described above. It is however practically difficult to keep the slurry from 40 through 90° C. in a die by the above-described heating procedure, because it is quenched when injected into the die, as described above. This has been experimentally confirmed.
In order to solve these problems, a technique is proposed, e.g., by Patent Document 4 which discloses a structure provided with a heating member for heating the slurry in such a way to encompass a die cavity (molding space).    [Patent Document 1] Japanese Patent Publication No. 1-54167 (Claims)    [Patent Document 2] Japanese Patent Laid-Open No. 6-182728 (Claim 1)    [Patent Document 3] Japanese Patent Publication No. 2-13924 (Claims and Page 3)    [Patent Document 4] Japanese Patent Publication No. 1-54168 (Claims)